edit.html.erb

<%= form_for @timesheet, :builder => TimesheetFormBuilder do |timesheet_form| %>
  <%= timesheet_form.fields_for :worklogs do |worklog_form| %>
    <%= worklog_form.text_field :monday %>
  <% end %>
<% end %>

timesheet_form_builder.rb

class TimesheetFormBuilder < ActionView::Helpers::FormBuilder
  def text_field(method, opts)
    #Get a reference to the model object
    object = @template.instance_variable_get("@#{@object_name}")
    value = object.send(method)
    (opts ||= {}).merge!(value: value + 'h')
    super(method, opts)
  end
end